The glass industry is a major user of natural gas. Natural gas accounts for 70 percent of the industry's 300.times.10.sup.12 Btu annual energy consumption. Almost 50 percent of this energy, or approximately 150.times.10.sup.12 Btu/yr., is natural gas consumed in melting furnaces. A glass melting furnace typically consists of a melter and a refiner. The raw materials, referred to as batch, are fed into the melter by the batch feeders. The batch consists primarily of sand, limestone, and soda ash. The melter is a refractory-lined pool of unmelted batch and molten glass. Several burners firing over the pool heat and melt the materials. The molten glass then flows through the throat to the refiner to allow trapped bubbles to rise, thus ensuring a defect-free product.
The melter operates at temperatures near 2800.degree. F. At this high temperature, heat losses are large and material properties limit the amount of insulation that can be used. Additionally, the sensible heat in the flue gases is large. To recover this heat, regenerators are used in most glass furnaces to preheat the combustion air. These regenerators consist of a latticework of brick called checkers. The flue gases are passed through one checker which they heat. When this checker is sufficiently hot, the valve is switched, diverting the flue gases to the second checker and drawing the combustion air through the first checker. The first checker then serves to preheat the combustion air. This cycle is repeated about every 20 minutes with flue gas temperatures for each checker varying with time. Checker exhaust temperatures vary from 600.degree. to 1200.degree. F.
For a typical regenerative furnace energy inputs vary from about 4.times.10.sup.6 to 7.times.10.sup.6 Btu per ton of glass. For a 200 ton-per-day unit, a typical energy requirement would be about 6.times.10.sup.6 Btu per ton. Only about 20 to 30 percent of the input is required to provide for the thermochemical reactions involved and for heating the glass. The balance of the heat input is lost in the flue gases and through furnace surfaces as convection and radiation losses.
One area for improvement of glass melting furnace energy utilization is greater recovery of heat from flue gases. Currently, essentially all glass furnaces use regenerators to recover heat from the flue gases to preheat combustion air. However, exhaust temperatures from the regenerator are still sufficiently high to provide not only preheating of glass batch materials but to perform additional heating functions as well.
Attempts to preheat batch agglomerated in the form of pellets or briquettes have been made. However, the agglomeration process results in high capital or operation and maintenance costs. Because of these disadvantages, fluidized beds have been considered for batch preheaters. With a fluidized bed preheater, hot flue gases from a regenerator associated with the melting furnace are ducted to rise vertically through the loose glass batch. The gas velocity is chosen such that the drag on each particle is equal to its weight, and hence each particle is suspended in the gas flow. Fluidized beds have other properties which recommend them for preheating, including high heat transfer rates, ease of materials handling, and high effective thermal conductivity.
In addition to the need for reducing energy use, increasingly strict environmental controls require not only the reduction of air pollution by exhaust gases, but also the elimination of particulate pollution. The fluidized bed will act as a filter and help to achieve this objective.
The provision of steam injection in a system for preheating glass batch materials would also be helpful not only as a processing aid but also to reduce pollution. Steam could also be generated within the system for this process and other purposes such as room heating.
Accordingly, it is a primary object of the present invention to conserve energy in the making of glass.
Another object of the present invention is to reduce particulate and gaseous pollution in the operation of glass furnaces.
A further object is a more efficient glass batch preheater providing an output at high temperature.
A still further object is a glass batch preheating system having a low initial cost and low operating and maintenance costs.